The present invention relates to a method and apparatus for preventing the detrimental spray emitted by a spraying nozzle from reaching the surface of a paper web.
Coating a paper sheet with a pigment slurry is performed by applying a coat of uniform quantity and quality to the surface of a paper web, after which excess water is removed from the web by means of dryers. The coating process steps can be divided into the application step, which comprises spreading of the coating mix onto the surface of the moving web, the control of the final coat weight and the smoothing of the coat surface.
A technique suitable for use in coating a paper web is the so-called spray-coating method that is described, e.g., in the international patent application publication WO 97/13035. In the spray-coating technique, the coating mix is administered to the paper sheet surface by application from spraying nozzles in which the coating mix is atomized into small droplets prior to spraying onto the moving paper web. The atomization of the coating mix in the spraying nozzle may be based on, e.g., a rotary motion, high-pressure atomization of a liquid coating or high-pressure air-assist/blast nozzles. The coat quality can be controlled by adjusting the distance between the spraying nozzle and the web, the spray-jet velocity and the mass rate of spraying. Generally, there must be disposed a number of spraying nozzles in the cross-machine direction to the web travel, because the coverage of the spraying jet emitted by a single nozzle is usually narrower than the web width. Typically, there is disposed a number of staggered nozzle rows across the web, perpendicular to the web travel, since a single row of nozzles extending across the web usually fails to provide a coating of a sufficiently high quality. In order to collect the coating mist occurring in the coating process and to recover the same from the air, the spraying nozzles are located in a closed space sealed from the environment, such as an enclosing hood having an exit opening made thereto for coating mist removal.
The maintenance of spraying nozzles and coating mix feed lines includes washing. Conventionally, the washing operation is carried out by injecting washing liquid into the feed line and spraying nozzles to be washed. Preferably, the washing operation should be arranged to be feasible also in a running papermaking machine in a manner not causing disturbance to the coating process of the paper web. Currently, this is possible if only a small quantity of the nozzles is washed at a time, whereby the deterioration of the coat quality remains essentially negligible. Moreover, the washing liquid emitted from the spraying nozzles must be prevented from reaching the surface of the paper web being coated.
It is an object of the present invention to provide a novel type of assembly and method for preventing a washing liquid spray or other detrimental spray from being directed toward the paper web.
The goal of the invention is attained by inserting during the washing operation to in front of the spray emitted by the nozzle a baffle that prevents the washing liquid passed via the nozzles from reaching the paper web.
More specifically, the assembly according to the invention is characterized by what is stated in the characterizing part of claim 1.
Furthermore, the method according to the invention is characterized by what is stated in the characterizing part of claim 9.
The invention offers significant benefits.
By virtue of the invention, the spraying nozzles employed in the coating process of a paper web can be washed also in a running papermaking machine without causing disturbance to the coating process. Moreover, the embodiment according to the invention is uncomplicated and cost-advantageous to implement as well as easy to automate.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are intended solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims.